1. Field of the Invention
This invention relates to an ink jet recording apparatus and method, in which a plurality of recording head units which are detachably mounted each having a plurality of nozzles are employed, and the recording head units and a recording medium are moved relative to each other, to perform a monochromatic printing operation and a color printing operation, and more particularly to an ink jet recording apparatus and method, in which the shift in position of a print, in the direction of arrangement of nozzles, of each recording head unit is corrected.
2. Description of the Prior Art
In the case of an ink jet recording apparatus in which a plurality of recording head units which are detachably mounted on a carriage each having a plurality of nozzles, which carriage is moved with respect to a recording medium, prints formed by the recording head units are put one on another. However, if the prints are shifted in position (hereinafter referred to as "print shifts", when applicable) for instance because the recording head units are mounted on the carriage with dimensional errors, then the resultant print is lower in picture quality as much. If a color printing operation is carried out with recording head units having mounting errors, the prints are shifted in position; that is, it is impossible to accurately put different color ink droplets one on another, which results in the mixture of colors, and the nonuniformity of color. This problem of picture quality is more serious according as the recording head units are improved in resolution.
If the recording head units on a printer are semi-permanently long in service life, then, their mounting positions can be adjusted only when shipped out of the factory; that is, if the positions are adjusted once, then it is no longer required to perform the adjustment again. However, in replacement of the recording head units which are replaceable units, the new ones are not always set at the same positions for instance because of their mechanical tolerances and mounting errors, and therefore the above-described print shift is liable to occur.
On the other hand, the recording head must be installed with higher accuracy in proportion to the increasing resolution thereof. In the recording system in which a plurality of recording head units are mounted individually, it is considerably difficult to mount them without positional shift.
In order to mount the recording head units in position with high accuracy, an adjusting mechanism may be added to the carriage. However, this method is disadvantageous in the following points: The carriage body is increased in manufacturing cost and it becomes bulky. In addition, it is difficult and troublesome to adjust each of the recording head units; that is, whenever the adjustment is performed, it is necessary to print with the recording head units thus adjusted to ensure whether or not the adjustment is satisfactory. Thus, the adjustment takes a lot of time and labor.
A multi-nozzle ink jet recording apparatus in which a plurality of recording heads each having a plurality of nozzles are mounted on a carriage which moves relative to a recording medium, and the positional shift of prints in the direction of arrangement of the nozzles (hereinafter referred to as "a nozzle arrangement direction", when applicable) between the recording head units is corrected, has been disclosed, for instance, by Unexamined Japanese Patent Publication (Kokai) Sho-62-77951/(1987).
The multi-nozzle ink jet recording apparatus is designed as follows: Each of at least (N-1) recording head units (where N is the number of recording head units mounted on the carriage), has nozzles the number of which is larger than a predetermined number of nozzles which are actually used for printing, so as to adjust the prints in the nozzle arrangement direction between the recording head units; that is, the print shift is corrected by selecting a predetermined number of nozzles in a proper position.
In the above-described conventional method, the number of nozzles selected is constant (for instance 24) at all times, and with the maximum positional shift taken into account, nozzles more than the constant must be provided; that is, extra nozzles must be provided. Hence, in the case where the recording head units mounted are less in positional shift, the number of nozzles which are not actually used is increased as much.
On the other hand, it is necessary to provide more extra nozzle in proportion to the increasing resolution of a recording head, and therefore the number of nozzles which are normally not used for printing is further increased. This fact results in an increase in manufacturing cost of the recording head, and in insufficient use of the recording ability of the recording head.
In the above-described multi-nozzle ink jet recording apparatus, the positional shift is detected by the following method: That is, the apparatus is operated to print actually, and the resultant print is used for correction of the positional shift. A prior art concerning the method has been disclosed by Unexamined Japanese Patent Publication (Kokai) No's Sho-62-109657/(1987) and Hei-2-243373/(1990). Those applications relate to a technique that, in a printing operation, prints are aligned with each other in position in a reciprocation mode. In the technique, printing is actually performed, to detect the positional shift of the prints, thereby to correct it. More specifically, a test printing operation is carried out with the printing timing changed for the "go" and "return" movements of the carriage until the print shift is eliminated. This conventional technique is applicable also to the correction of a print shift in a color printing operation.
In the conventional technique, the carriage is reciprocated several times until the print shift is eliminated, and a value considered to be optimum is inputted. If the print shift is large, the carriage must be reciprocated many times, and the printing also must be carried out many times. As a result, one sheet of recording paper may be used up only for correction of the print shift. That is, the conventional technique is disadvantageous in that it is not economical.